Authentication is precisely the problem DNSSEC is supposed to solve. And authentication is part of transport security.
TLS usually does provide authentication. The certificate is signed by a chain that leads up to a trusted CA, and content of the stream is authenticated using an AEAD encryption algorithm. But RFC 9539 specifically doesn't require the certificate to be signed by a CA (and recommends using a self signed cert) or the client to verify the authenticity of the certificate used by the server.
This means that an active MitM could return whatever it wanted to the recursive resolver. Pulling that off is admittedly more difficult than a MitM on a public wifi network at a coffee shop, but it's still a gap in security.
And no, you can't just use Web PKI to verify connections to authoritative DNS servers, because Web PKI relies on DNS to confirm domain ownership.
